Access technologies of various kinds, especially wireless, are becoming increasingly ubiquitous, e.g. in the shape of GSM (Global System for Mobile communication)/GPRS (General Packet Radio Service)/EDGE (Enhanced Data rates for GSM Evolution), WCDMA (Wideband Code Division Multiple Access)/HSPA (High Speed Packet Access), CDMA2000 (a cellular network standard based on Code Division Multiple Access), WLAN (Wireless Local Area Network), WiMAX (Worldwide Interoperability for Microwave Access) and soon LTE (Long Term Evolution). The mobile terminals or other types of user equipment (UE) match this multitude of access technologies by including ever more access interfaces to allow greater freedom and flexibility in the selection of access to use for each communication session.
To leverage the benefits of this growing flexibility it becomes important to have mechanisms in place for efficient control of the access selection, to ensure that a mobile node always uses its available access interfaces and access networks as efficiently as possible for the currently ongoing communication sessions. Circumstances to be taken into account include, e.g., the currently used applications, access network technologies and their properties, access network operators (and their relations to the user's home operator and/or the user), current network conditions (e.g. load), location, subscription restrictions, time of day, etc. Similarly, it is desirable to support a mobile node in discovering available accesses without requiring the mobile node to continuously scan for all accesses and thus using battery resources.
In SAE (System Architecture Evolution)/LTE, also known as EPS (Evolved Packet System), i.e. the future evolved 3GPP system, multi-access is a key element. Control of access selection (and access discovery) is recognized as an important aspect and has been assigned a dedicated work item in the 3GPP standardization process.
The mechanisms considered are based on policies and/or rules. The functionality provided by a policy/rule is instructions or guidance of which access to select or how to discover accesses given the specific circumstances (device context). A policy/rule typically defines contextual conditions when it applies (e.g. for a certain application, in a certain geographical region, a certain time of day and/or day of the week, the PLMN (Public Land Mobile Network) the UE is currently registered at, e.g. a visited PLMN during roaming, etc.) and access preferences. The access preferences may e.g. be a prioritized list of accesses, an indication of a specific only access, a list of prohibited accesses, or some other way of expressing how preferable certain accesses are.
There are three basic cases of access selection:                Selection between multiple 3GPP accesses, e.g. LTE, HSPA, WCDMA, GERAN (GSM EDGE Radio Access Network), GSM. This access selection case may also comprise the 3GPP2 (3rd Generation Partnership Project 2) access CDMA2000, but for simplicity it is referred to as “intra-3GPP access selection” or simply “3GPP access selection”.        Selection between 3GPP and non-3GPP access(es) (where non-3GPP accesses include e.g. WiMAX and WLAN). This access selection case is referred to as “extra-3GPP access selection”.        Selection between multiple non-3GPP accesses. This access selection case is referred to as “non-3GPP access selection”.        
Extra-3GPP access selection and non-3GPP access selection are also collectively referred to as “ANDSF based access selection”, because, as will be explained later, they are controlled by the functionality provided by the Access Network Discovery and Selection Function (ANDSF). Accordingly, in the following extra-3GPP access selection and non-3GPP access selection are for reasons of simplicity also collectively referred to as “ANDSF based access selection”.
Policies and/or rules may be processed in the network, e.g. in the PCRF (Policy and Charging Rules Function) or, in the context of access selection, for example in the newly introduced functional entity ANDSF, which is responsible for extra-3GPP access selection as well as non-3GPP access selection, i.e. access selection between 3GPP accesses and non-3GPP accesses and between different non-3GPP accesses. The ANDSF can be distributed between the mobile node or UE and the network. In the network the ANDSF can be located both in the home network and in the visited network. The ANDSF functionality in the UE is referred to as ueANDSF, the ANDSF functionality in the home network is referred to as hANDSF, and the ANDSF functionality in the visited network is referred to as vANDSF. The network ANDSF can be located in an entity inside (i.e. as an integral part of) or can be associated with the PCRF. It is also possible that there will be ANDSF related functionality in non-3GPP access networks, e.g. for provision of access properties as input data to the access selection process. Such possible ANDSF related functionality in non-3GPP access networks is herein tentatively labeled n3aANDSF. Further information on the introduction of the ANDSF in the 3GPP SAE architecture, as well as the related information flows, can, e.g. be found in “3GPP TS 23.402 v8.1.1, ‘3rd Generation Partnership Project; Technical Specification Group Services and System Aspects; Architecture enhancements for non-3GPP accesses (Release 8)’, March 2008”.
For 3GPP access selection, i.e. selection between different 3GPP accesses that interwork on radio access network (RAN) level (such as LTE, WCDMA/HSPA and GERAN, and possibly the 3GPP2 access CDMA2000), the access selection functionality is separate from the ANDSF. This access selection function is network based and is typically located within the RANs, e.g. E-UTRAN (Evolved Universal Terrestrial Radio Access Network), GERAN, UTRAN (Universal Terrestrial Radio Access Network), CDMA2000-RAN, and possibly partly also in the MME (Mobility Management Entity) and/or SGSN (Serving GPRS Support Node). Potentially, processing of access selection related policies and rules may take place also in this access selection function.
The ways the 3GPP access selection functionality controls the access selection of the UEs differ between connected-mode and idle-mode UEs. Here, “connected-mode UEs” refers to UEs in connected mode, which have a signaling connection established to a RAN.
Access selection for connected-mode UEs is manifested as handovers between different accesses. This is, e.g., explained in “3GPP TS 36.300 v8.4.0, ‘3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2 (Release 8)’, March 2008” and in “3GPP TS 36.331 v8.1.0, ‘3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA) Radio Resource Control (RRC); Protocol specification (Release 8)’, March 2008”. The handovers are network controlled, but aided by measurement reports (e.g. received signal power from neighbor cells) from the UEs. The network sets the trigger criteria for the UEs' measurement reports (through broadcasting of thresholds and/or by sending UE specific criteria to individual UEs) and also determines when and to which cell (and access network) a handover should be performed.
For idle-mode UEs access selection is realized as cell reselection, i.e. repeatedly evaluating the best cell to camp on, as well as access technology (or Radio Access Technology, RAT) selection. This is, e.g., explained in “3GPP TS 36.304 v.8.1.0, ‘3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); User Equipment (UE) procedures in idle mode (Release 8)’, March 2008”. The network broadcasts (in the system information) the signal strength threshold levels determining when the UEs should evaluate neighbor cells for cell reselection. The network can also provide RAT and carrier frequency priorities in the broadcast system information and optionally in a UE specific signaling message when the RRC (Radio Resource Control) connection to the RAN is released.
For initial selection of PLMN and preferred access technology, e.g. when the UE is powered on, the UE is typically guided by information stored on the SIM card (SIM: Subscriber Identity Module) or USIM card (USIM: Universal Subscriber Identity Module). This is, e.g., explained in “3GPP TS 36.304 v.8.1.0, ‘3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); User Equipment (UE) procedures in idle mode (Release 8)’, March 2008” and in “3GPP TS 23.122 v8.1.1, ‘3rd Generation Partnership Project; Technical Specification Group Core Network and Terminals; Non-Access-Stratum (NAS) functions related to Mobile Station (MS) in idle mode (Release 8)’, March 2008”). This information originate from the operator (i.e. the operator with which the SIM/USIM card is associated), but may be complemented with information from the user. The user can also choose to make the initial selection purely manual.
An advantage of keeping the 3GPP access selection functionality distinct from the ANDSF based access selection functionality is that both selection functionalities can be optimized in this way for different time scales and/or interfaces. As an example, ANDSF communication may take place on a long time scale, possibly minutes, hours or even months. Thus updates of the rules can be comparatively seldom and may be transported easily by the IP protocol. In contrast, 3GPP access selection may be optimized for a synchronization of communications based on timeslots of the access systems, typically on the order of several milliseconds. In this way, a UE can for example perform measurements on one access systems during short intervals in a transmission over a different 3GPP access system. This allows the preparation of inter-system handovers even for a UE with only a single transceiver but requires elaborate interfaces.
Access selection and access discovery are not restricted to mobile or wireless nodes/terminals. They are equally applicable for so called “user networks”. The term user network (UN) refers to one or more inter-connected user devices that can access a network via one or more access technologies. Examples of a single-device user network are a cellular phone or a laptop, while an example of a multi-device user network is a Personal Area Network (PAN). Although only the term UE will henceforth be used in this document, it should be interpreted as either a single device, i.e. a mobile terminal, or a multi-device user network.
The problem with the existing solution is that because the two access selection mechanisms are separate, access selection loops may occur in some situations. To illustrate this problem consider the following example.
In a certain situation (defined by context parameters, e.g. active applications, time of day, etc.) the 3GPP access selection functionality has the following prioritized access preference list (most preferred first):
1. WCDMA
2. LTE
3. GERAN
Reasons for this priority order may include policies for load balancing or steering of certain applications to certain access networks (e.g. running voice calls over circuit switched WCDMA bearers).
In the same situation the ANDSF based access selection functionality has the following prioritized access preference list (most preferred first):
1. Corporate WLAN
2. 3GPP domain if 3GPP access=LTE is available
3. WiMAX
4. 3GPP domain if 3GPP access=WCDMA or GERAN is available
Initially we assume that the concerned UE is connected to the 3GPP domain. Within the 3GPP domain the 3GPP access selection mechanism directs the UE to WCDMA (even if LTE is available in the current location), because this is the highest priority 3GPP access (in the given situation).
The UE detects an available WiMAX access and the ANDSF based access selection mechanism determines to leave the 3GPP domain and hands over the UE to the WiMAX access, because it has a higher priority than a 3GPP domain represented by WCDMA.
While the UE is connected to WiMAX it scans for higher priority access networks. It detects LTE but no corporate WLAN and thus, in accordance with its prioritized access preference list, the ANDSF based access selection functionality hands over the UE to LTE in the 3GPP domain.
Once connected to the 3GPP domain the above described mechanisms used by the 3GPP access selection functionality kicks in and directs the UE to WCDMA, thereby forming a potentially endless access selection loop.
Accordingly, there exists a need to provide improved techniques for access network selection in a multi-access network environment which provides multiple independent access selection processes.